Harden OIDC, add SVG sanitization, improve form UX and security defaults

Remove PKCE plain method support (S256 only), enforce openid scope requirement,
filter to supported scopes, strip reserved claims from custom claims as
defense-in-depth, sanitize SVG icons with Loofah, add global input padding,
switch session cookies to SameSite=Lax, use Session.active scope, and remove
unsafe-eval from CSP.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/models/oidc_authorization_code.rb

@@ -9,7 +9,7 @@ class OidcAuthorizationCode < ApplicationRecord
 
   validates :code_hmac, presence: true, uniqueness: true
   validates :redirect_uri, presence: true
-  validates :code_challenge_method, inclusion: {in: %w[plain S256], allow_nil: true}
+  validates :code_challenge_method, inclusion: {in: %w[S256], allow_nil: true}
   validate :validate_code_challenge_format, if: -> { code_challenge.present? }
 
   scope :valid, -> { where(used: false).where("expires_at > ?", Time.current) }